1. Field of the Invention
This invention relates generally to the field of electronic filters. More particularly, the present invention provides a microwave filter having a temperature compensating element.
2. Description of the Related Art
Microwave filters are known in this art. A microwave filter is an electromagnetic circuit that can be tuned to pass energy at a specified resonant frequency. The filter is used in communications applications to filter a signal by removing frequencies that are outside a bandpass frequency range. This type of filter typically includes a housing with an input port and an output port. Internally, a typical microwave filter includes an array of interconnected filter cavities. In many microwave filters, the resonant frequency of the filter may be adjusted with tuning screws that typically protrude through the housing and into each filter cavity. One such filter type is a coaxial microwave filter.
FIG. 1 is a cross-sectional view of a known coaxial microwave filter 10. The coaxial filter 10 includes a housing wall structure 12 that defines a plurality of interconnected filter cavities 13, and a filter lid 14 that is fixedly mounted to the housing wall structure 12 to cover the cavities 13. Each filter cavity 13 includes a resonator rod 16 projecting upward from a bottom wall of the housing wall structure 12, typically at the center of the cavity 13, and a tuning screw 18 mounted through the filter lid 14 opposite the resonator rod 16. The tuning screw 18 may be adjusted to extend into a bore 19 in the center of the resonator rod 16. It should be understood, however, that although only one cavity 13 is shown in FIG. 1, the filter 10 typically includes an array of cavities 13 that are interconnected through openings, such as irises, in the cavity walls. It should also be understood that a three dimensional view of the cavity 13 would show the resonator rod 16 and tuning screw 18 in the center of an open cavity 13, i.e., there is open space within the cavity 13 on all sides of the resonator rod 16.
The electrical resonance of each cavity 13 in the filter 10 is determined by the combination of the length of the resonator rod 16, the size of the cavity 13, the size of the gap 20 between the resonator rod 16 and the filter lid 14, and the insertion depth of the tuning screw 18 into the resonator rod 16. The insertion depth of the tuning screw 18 into the resonator rod 16 can, therefore, be adjusted to change the resonant frequency of the filter 10.
The resonant frequency of the filter 10 may be undesirably altered, however, by minute changes in the size of the cavity 13 resulting from thermal expansion or contraction of the housing material and the resonator rod 16 during a change in ambient temperature. This drift in frequency with temperature may be reduced by using different materials for the resonator rod 16 and the housing 12. For example, the filter lid 14 and housing wall structure 12 may be manufactured from aluminum, while the resonator rod 16 is made from some other type of metal or possibly a ceramic material. Even with such a design, however, some amount of temperature-dependant frequency drift typically remains.
A microwave filter having a temperature compensating element includes a housing wall structure, a filter lid, a resonator rod, a tuning screw and the temperature compensating element. The housing wall structure defines a cavity. The filter lid closes the cavity. The resonator rod is within the cavity. The tuning screw is adjustably mounted through the filter lid and has a portion that protrudes into the cavity and is coaxial with the resonator rod. The temperature compensating element is joined to the filter lid or the housing and forms a bimetallic composite with the filter lid or housing that deforms with a change in ambient temperature.